The invention relates to the bending of metal pipe, as for example to the bending of steel pipe.
A current practice in the hot-bending of large-diameter steel pipe, e.g., 4-inch diameter and up to 42-inch diameter, is to fill a straight pipe length with sand, subjecting the filled pipe to heat-soaking at an elevated temperature, supporting the heat-soaked length with one end clamped to and end of an arcuate forming die, and applying bending force to the free end of the pipe to the extent of a prescribed angle of bent wrap around the die, the die having a sectional radius of concave curvature which equals or is slightly greater than the outside curvature (O.D.) of the pipe. Such a process and bending die are described in Crippen, U.S. Pat. No. 3,456,468.
Generally speaking, it has been considered that five pipe diameters is the minimum curvature radius to which such pipe can be acceptably bent, i.e., without causing the cross-section of the pipe to change from circular to generally elliptical, and without causing local corrugations to develop in the concave side of the bend. The Pipe Fabrication Institute has expressed this limitation at paragraph 4.1 of its PFI Standard ES-24 (April 1975), stating:
"When the radius of a bend is 5 nominal pipe diameters or greater, and the ratio of the nominal interior diameter to the nominal wall is 35 or less, the difference between the maximum and minimum diameters shall not normally exceed 8% of the average measured outside diameter of the straight portion of the pipe. Where special operating conditions or code provisions require an ovality less than 8% it may be necessary to use larger radii or heavier pipe walls to achieve such requirements."